This invention relates to a friction composition comprising a mass or groups of glass fibers uniformly dispersed in a body bonded together by a cured organic binder and also preferably containing a particulate heat-containing material and a binder modifying material. Shaped bodies of such compositions are used as the friction element in brake systems, clutches and other movable devices which employ a friction element. Familiar applications would include brake pads or linings in automotive disc or shoe brake systems and clutch plates.
Friction compositions intended for use in automotive and other vehicles for brake or clutch application must withstand severe operating temperatures and pressures under repeated application without physical failure (disintegration) or deterioration in performance, (brake fade, etc.). The conventional brake lining or pad in use in the automotive industry uses asbestos as the friction material bound in a thermoset resin, and usually contains additional materials such as mineral fillers, metallic powders, antioxidants and friction modifiers. Asbestos linings or pads have been generally satisfactory in the past; however, increasingly severe operating standards due to stricter safety regulations, increased vehicle weight due to safety equipment and options, etc. and higher performance vehicles have made it currently desirable to seek alternative compositions to reduce or eliminate the asbestos content of brake linings in automobiles.
Because of the physical strength of bodies of glass fibers bonded together with resinous binders, such compositions have been suggested in the prior art for use as reinforcing layers for composite, multiple layer friction materials for clutches, brakes and the like. U.S. Pat. Nos. 2,158,337, 3,365,041, 3,429,766 and 3,520,390 disclose the use of woven glass fabric as a reinforcement for conventional asbestos or metallic brake or clutch linings. In addition, U.S Pat. Nos. 2,554,548 and 2,158,337 suggest the use of glass fibers with conventional brake materials (asbestos) in the brake lining, wherein the glass fibers are used to physically reinforce the composite lining and/or to act as the friction material itself.
Beyond these suggested uses in prior patents, glass fibers have not been widely used in the makeup of friction materials such as brake pads or linings. This may be because the wearing properties and coefficient of friction of glass fibers have been not fully appreciated.
By way of further illustration of the known prior art, typical asbestos-based friction products, although effective and adequate under ordinary service conditions, often exhibit appreciable losses in braking effectiveness and diminished wear life when subjected to the rigors of high energy service conditions. For instance, the commercially available asbestos based brake pads or linings soon expend their effectiveness and begin to fade with a loss of uniform braking friction properties and decrease their resistance to wear when exposed to high pressure and temperature conditions encountered in extreme high energy braking service conditions. Such extreme conditions of temperature and pressure are encountered more often with disc type automotive brakes where the total braking area is usually smaller than in shoe type brakes.* FNT (*Brakes for automotive vehicles are of two basic types: (1) the internal expanding type in which arcuate shaped brake linings on a pair of non-rotating shoes are forced into engagement with a moving brake drum. (2) Disc type brakes in which a moving disc or rotor is clamped between a pair of opposed pads of friction material. The arcuate linings or the pads are conventionally asbestos based as described while the drum or disc is cast iron or other relatively rigid, heat conductive material.)
Conventional pads for disc brakes and brake linings are composed primarily of asbestos fibers in an organic binder matrix which is usually a thermosetting resin or a mixture of thermosetting resin and a heat stable rubber. Because considerably higher temperatures are generated in disc brakes than in drum brakes, the organically-bound friction materials in the pads are more likely to disintegrate due to the thermal degradation of such binders, resulting in inferior frictional characteristics, promoting fade and resulting in increased wear. The higher temperatures also increase the activity of the binder with other materials, for example the disc iron, to form liquid or other low friction materials. Thus, the wear on both the pads and rotor or disc is accelerated at high temperature operation. This high temperature failure is believed to be due to depolymerization or thermal decomposition of the binder itself, which, in addition to the deleterious effects described above, causes the friction material iself (asbestos and possibly other particulate additives) to become loose and thus move under the high pressure of applied brakes, causing deformation of the pad itself. The result is an accelerated wear rate, possible adhesion of the pad to the disc surface, and generally decreased coefficient of friction which appears to the user as brake fade.
While disc type brakes place more severe operating temperatures and pressures on the friction materials than do shoe brakes, they are generally more efficient and are being used with increasing frequency by the automotive industry. Therefore, the need for friction materials capable of good performance at such temperatures and pressures which is economically attractive has increased, particularly in view of increasingly rigid safety and endurance requirements now being faced by the industry.